Optical head and optical disk apparatus

Details Optical head and optical disk apparatus Optical head and optical disk apparatus

1998-11-03

2001-08-14

Huber, Paul W. (Department: 2651)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S112230

Reexamination Certificate

active

06275453

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to an optical head and an optical disk apparatus, and more particularly to an optical head and an optical disk apparatus, whose beam spot is rendered minute.
2. Description of Related Art
In optical disk apparatus, realization of both higher density and larger capacity of optical disks from compact disks (CD) to digital video disks (DVD) is proceeding, and optical disk apparatuses are increasingly requested for larger capacity in order to meet the tendency of computers toward higher performance and that of displays toward higher definition.
The recording density of an optical disk is basically limited by the size of a beam spot formed on a recording medium. When light is condensed by an objective lens, a diameter (beam spot size) D
½
at which the optical intensity of the beam spot becomes ½ is given by the following equation (1), and the track width becomes substantially equal to this size.
D
½
=k&lgr;/(n·NA)  (1)
where k: Proportionality constant (normally about 0.5) depending on the intensity distribution of the beam,
&lgr;: Wavelength
n: Refractive index (normally air, nearly 1) of medium at the position of beam spot,
NA: Numerical aperture of the objective lens.
Since the NA of objective lenses used with conventional optical disks is about 0.5, D
½
is nearly equal to the wavelength. Also, as can be seen from equation (1), the use of a shorter wavelength or objective lenses of larger NA is effective to obtain a minute beam spot, and development efforts have been made respectively. In DVDs, the wavelength was shortened to 0.65 &mgr;m, and the NA of the objective lens was raised from 0.45 in the case of CD to 0.6 thereby providing a density roughly four times higher than CD in DVD. As for the wavelength, a green or blue luminous has further been vigorously developed. On the other hand, as for NA, when it exceeds 0.6, the influence of signal intensity fluctuation due to tilt of the optical disk becomes significant. Thus, it is difficult to increase the NA higher than 0.6 in the conventional optical recording system which is performed using a plastic substrate. Therefore, current optical storage development is shifting toward condensing light on a recording layer formed on a plastic substrate without passing the light through the plastic substrate.
In the optical recording systems that directly condense light on a recording layer, the following two systems using near field optics have been recently proposed for radically reducing the beam spot size. These systems have been both obtained by applying the high-resolution techniques of microscopes to optical recording.
The first system employs near field optics for recording in which light is emitted from the tip end of an optical probe whose tip end has been polished to a small tapered shape (several tens of nanometers or less). This system has many problems such as difficult and unstable working of the probe, susceptibility of the probe to mechanical shocks, short life, low light utilization efficiency of 1/1000 or less, and requires many improvements to put it to practical use.
The second system places a hemispherical lens (Solid Immersion Lens (hereinafter, abbreviated to “SIL”)) consisting of a transparent medium having a high refractive index near the focus of an objective lens to thereby form a minute beam spot at the central portion of the bottom of the SIL for performing optical recording, and can be considered to be a technique having comparatively higher feasibility than the first system. Since the wavelength of light becomes shorter in inverse proportion to the refractive index of the SIL within it, the beam spot also becomes smaller in proportion thereto. The majority of light condensed at this beam spot is totally reflected toward the hemispherical surface of the SIL, and some portion thereof is emitted in the neighborhood of the beam spot outside of the SIL as near field light. If a recording medium having nearly the same refractive index as the SIL is arranged in the neighborhood (at a sufficiently smaller distance than the wavelength of light), the near field light enters this medium and propagates within the medium. By using this light to record on the medium, it becomes possible to perform high-density recording. Since, however, an aberration of the objective lens remains present, it is necessary to maintain the aberration of the objective lens sufficiently low. The light condensing system using this SIL has two types to be described below.
FIG. 13
shows an optical head of the first type. This optical head
50
comprises an objective lens
52
for condensing a collimated beam
51
, and a hemispherical SIL
54
arranged so that a bottom face
54
a
thereof intersects convergent light
53
from the objective lens
52
. When the collimated beam
51
is incident on the objective lens
52
, the collimated beam
51
is condensed by the objective lens
52
, the convergent light
53
from the objective lens
52
is incident on the hemispherical surface
54
b
of the SIL
54
, and is condensed at the center of the bottom face
54
a
of the SIL
54
to form a beam spot
55
. The diameter of the beam spot
55
at the optical head
50
is reduced in inverse proportion to the refractive index of the SIL
54
. When the recording medium
56
is brought close to the beam spot
55
, the near field light in the neighborhood of the beam spot
55
is incident on the recording medium
56
as propagation light.
FIG. 14
shows an optical head of the second type. This optical head
50
comprises an objective lens
52
for condensing a collimated beam
51
, and a bottomed SIL
54
arranged so that the bottom face
54
a
thereof intersects convergent light
53
from the objective lens
52
. The SIL
54
is arranged so as to refract the convergent light
53
from the objective lens
52
and further condense it. The SIL
54
is constructed such that the collimated beam
51
is condensed in a distance of r
(r is radius of SIL) from the center
54
c
of the hemispherical surface
54
b
(called “Super SIL Structure”), whereby it is possible to have small spherical aberration due to the SIL
54
, to raise the numerical aperture within the SIL
54
to n times that of the objective lens
52
shown in
FIG. 13
, and further to make the beam spot
55
minute. That is, the beam spot can be rendered minute as shown by the following equation (2):
D
½
=k&lgr;/(n·NAi)=&lgr;/(n
2
·NAo)  (2)
where NAi: Numerical aperture within SIL
54
NAo: NA of incident light on SIL
54
However, NA of the incident light on this Super SIL
54
, that is, the maximum value &thgr;max of the incident angle &thgr;, is inversely related to the refractive index n of the SIL
54
, and the two cannot be made independently large.
FIG. 15
shows the relationship between the refractive index n of Super SIL
54
and NAo, obtained by Suzuki in #0C-1 of Asia-Pacific Data Storage Conference (Taiwan, '97, 7) (hereinafter, referred to as “First conventional example”). As can be seen from
FIG. 15
, when the refractive index n of the SIL is continuously raised, the maximum value NAomax which the NAo of the incident light can take gradually becomes smaller. This is because when the NAo increase s over the maximum value NAomax and the incident angle becomes larger, the beam spot
55
at the position of the recording medium
56
becomes wider because the light does not pass through the SIL
54
, but becomes directly incident on the recording medium
56
. When, for example, the refractive index n=2, NAomax is 0.44, and the product n·NAomax is within a range of 0.8 to 0.9. This is the theoretical limit, and in reality is a smaller value (0.7 to 0.8).
Concerning the condensing experiment using the Super SIL, B. D. Terris et al reported in Appl. Phys. Lett. Vol. 68 ('96), P. 141 (hereinafter, referred to as “Second conventional example”). According to this report, a Super SIL having a refractive index n=1.83 i

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